Among the recent developments in the field of the grinding of metals has been the use of the so-called "sol-gel alumina" abrasive particles. In the Leitheiser U.S. Pat. No. 4,314,827, a description is given of a form of alumina abrasive having a microcrystalline structure of crystallites obtained by the sintering of a sol gel form of alumina; the patent describes the formation of an abrasive wheel by incorporating such a sintered sol gel alumina abrasive in a phenolic resin bond. The abrasive obtained by the sintered sol gel method described in the Leitheiser patent is a calcium ion-free and alkali metal ion-free, dense, non-fused, synthetic, aluminum oxide-based granular abrasive mineral, having a substantially homogeneous microcrystalline structure of randomly-oriented crystallites of alumina and a modifying component, wherein the alumina is the dominant continuous phase.
This form of alumina abrasive is being used in a number of products, including, for instance, the ceramic described in the Schwabel U.S. Pat. No. 4,744,802 which describes a method of forming the sol gel alumina abrasive particles in which a nucleating agent is added to a sol gel of alpha alumina monohydrate.
Similarly, the patent of Cottringer et al U.S. Pat. No. 4,623,364 teaches the use of seed material to enhance alumina gels in the formation of aluminous abrasive grits, while the patent of Narayanan et al U.S. Pat. No. 4,741,743 describes combining sintered gel alumina abrasive grits with fused alumina grits. Also, the patent of Haynes U.S. Pat. No. 4,800,685 teaches the use of sol gel alumina abrasive grain in a matrix to grind cast iron.
It is clear, then, that the sol gel aluminous abrasive particles (particularly the seeded sol gel aluminous abrasive particles) have demonstrated substantial advantages over other premium abrasives in broad areas of coated and bonded abrasive applications, since their introduction a few years ago. As has been stated above, such abrasives are generally made by drying and sintering a hydrated alumina gel, which may also contain varying amounts of additives, such as MgO or ZrO.sub.2. The dried material is crushed (either before or after sintering) to obtain irregular, blocky-shaped polycrystalline abrasive grits in a desired size range. The grits may later be incorporated in a bonded abrasive product, such as a grinding wheel or a segment. The Leitheiser patent, described above, discloses abrasive grits made by such a method in which the sintered grits contain irregular "snowflake" shaped alpha Al.sub.2 O.sub.3 crystals which are on the order of 5 to 10 microns in diameter. The spaces between the arms of a "snowflake" are occupied by other phases such as a finely crystalline alumina magnesia spinel.
The Cottringer et al patent, described above, discloses a sol gel method for the manufacture of aluminous abrasive grits, and products other than abrasive grits such as coatings, thin films, fibers, rods or small shaped parts, having enhanced properties. In that patent the conversion of the hydrated alumina to alpha alumina is facilitated by the introduction of seed material into the gel or the gel precursor prior to drying. This can be accomplished by either wet vibratory milling of the gel or gel precursor with alpha alumina media, or by the direct addition of very fine seed particles in powder or other form. To make abrasive grits the seeded gel is dried, crushed and fired. The abrasive grits so produced may be used in the manufacture of products such as coated abrasive disks and grinding wheels. Alternatively, to make shaped parts or rods, the material may be formed or molded as by extrusion before firing. In the case of extrusion; the rods formed are later cut or broken into appropriate lengths.
The Schwabel patent, described above, also discloses a seeded sol gel process for producing alpha alumina based ceramics useful as abrasive grain and ceramic shaped bodies. Such alpha alumina is obtained from alpha alumina monohydrate to which has been added a nucleating agent.
Once the gel has formed, it may be shaped, according to the patentee, by any convenient method such as pressing, molding or extrusion and then carefully dried to produce an uncracked body of the desired shape. If abrasive material is desired, the gel can be extruded, according to the disclosure, or simply spread out to any convenient shape and dried. After drying, the solid body or material can be cut or machined to form a desired shape or crushed or broken by suitable means, such as a hammer or ball mill, to form abrasive particles or grains.
Such seeded sol gel abrasives have a much firmer alpha Al.sub.2 O.sub.3 crystal structure and higher density than the Leitheiser-type unseeded sol gel material. The alpha Al.sub.2 O.sub.3 crystals of the seeded sol gel abrasives are submicron and usually on the order of about 0.4 microns and less, although somewhat coarser structure may result if the seeding is performed in a non-optimal manner or if the firing is at too high a temperature, or for too long a duration.
Other materials such as iron oxide, chromium oxide, gamma alumina, and precursors of these oxides, as well as other fine debris that will act as nucleating sites for the alpha alumina crystals being formed, can also be used as seeds to facilitate the conversion to alpha Al.sub.2 O.sub.3. As a rule of thumb, such seeding materials should be isostructural with alpha Al.sub.2 O.sub.3 and should have similar (within about 15%) crystal lattice parameters to work well.
When grinding the surface of hard chilled cast iron rolls, especially tougher Chromium-Iron rolls, it is customary to choose a grinding wheel that has a relatively soft grade since these tend to give the best performance. Such soft grade wheels are characterized by a sand blast penetration value (SBP) of up to about 5.1 and preferably up to about 3.1 mm and a weight per unit volume of less than about 2.8 and preferably less than about 2.20 g/cc. The "softness" of a wheel is affected by the resin bond chosen, the proportion of grit to bond material and the amount of porosity in the wheel. Typically suitable abrasive wheels consist of silicon carbide abrasive or blends of silicon carbide and alumina abrasive coupled with a low elastic modulus resinoid bond. The silicon carbide abrasive is needed to help the wheel penetrate into the hard surface of a chilled iron roll to remove metal from the surface at an acceptable rate. The low elastic modulus bond is necessary to prevent vibration from occurring between the wheel and roll surface and to prevent the development of chatter marks in the finished surface of the roll. Due to a higher toughness of chilled iron rolls made with a high chrome content, a silicon carbide/alumina abrasive blend is preferred. If silicon carbide alone were used on chrome-iron rolls, the abrasive would break down too rapidly, resulting in low metal removal rate. The stronger alumina helps to reduce wheel wear and improve abrasive penetration.
In most cases, steel mill production requires the grinding of a mixture of both high chrome and plain chilled iron with a single wheel. The abrasive selected in such cases is usually silicon carbide, which is found to be the most versatile. By using silicon carbide alone, abrasive penetration is ensured on the very hard chilled iron rolls. However, metal removal rate is compromised (reduced compared to the metal removal rate obtained with silicon carbide/alumina) on the tougher Chrome-Iron rolls. These and other difficulties experienced with the prior art abrasive wheels have been obviated in a novel manner by the present invention.
The present invention provides an abrasive wheel that is particularly suitable for roll grinding very hard materials. The abrasive wheel, has excellent wheel life and metal removal rate on chilled iron rolls and has chatter-free operation and improved roll shape control when used to finish chrome-iron rolls.
The abrasive wheel of the invention also is simple in formulation and has a long useful service life.